This invention relates to an apparatus and method for crushing sugar cane to improve the drainage and separation of the sugar juice from the cane or bagasse.
Sugar juice is removed from sugar cane by crushing the cane between crushing rollers. The cane is initially chopped into short lengths of about 20-50cm (called billets), or is shredded into a finely divided form. The billet or shredded cane is then crushed in a number of spaced apart mills.
A known type of mill is a three roll mill which consists of two bottom rolls next to each other and a top roll, the arrangement being that the centres of the roll shafts form a triangle. In this arrangement, a blanket of sugar cane or bagasse passes through the rolls in a generally horizontal or slightly inclined manner and juice is extracted during the crushing process.
Another known mill employs a pair of counter rotating rollers one above each other such that the sugar cane moves horizontally between the rolls.
Sometimes, the mill has one or more additional rolls in front of the main crushing or primary rolls. The additional rolls are there to push a blanket of compressed sugar cane or bagasse to the main crushing rolls. In doing this, the additional rolls usually crush some juice from the cane or bagasse, but this is incidental and the primary function of these rolls is to force the cane or bagasse to the crushing rolls.
When sugar cane is crushed in a factory, the cane passes through a number of separate mills. Each mill consists of the above mentioned two, three, four or more roller arrangement. The sugar cane is conveyed from one mill to the next mill for further crushing.
A common feature of all current conventional mills is that the lower roll functions to drain most of the sugar juice from the crush. This is primarily because of gravity effects causing the extracted sugar juice to move under the influence of gravity to the bottom roll. However, the top roll is also a crushing roll and extracted sugar juice also accumulates on top of the horizontally moving blanket of cane of bagasse.
To remove this juice, openings are sometimes present in the top roll through which the sugar juice can pass and drain from each end face of the roll. Such rolls are known as "lotus" rolls.
To improve the crushing efficiency of the cane or bagasse, it is known to groove both the top roll and the bottom roll to form a meshing-type effect.
For the bottom roll, it is known to cut additional juice grooves to a depth of about 25 mm in each or every second or third groove in the roll to facilitate juice drainage. The juice grooves provide a path for the juice to flow away from the crush.
However, a disadvantage with this is that the juice flows backwards against the direction of the roll. This counterflow of sugar juice against the roll surface movement is deleterious to the efficiency of juice removal. The frictional resistance of the roll surface in contact with the juice retards the flow of juice. The retardation effect increases as roll surface speed increases. Therefore the possible benefits of juice grooves are limited by flow retardation and may be cancelled out altogether as surface speed increases to the point that counter flow of juice ceases and the juice is carried forward with the roll.
This effect also severely limits the capacity of the mill, as capacity is directly related to the roll length, roll diameter and roll speed.
Increasing roll speed in conventional mills to above about 300 mm per second can result in a sharp decline in extraction efficiency.
Attempts have been made to place deep penetrating scrapers into the grooves in front of the roll in order to remove bagasse, but this has the unfortunate consequence of forcing the juice to be reabsorbed by the bagasse which again results in a decline in extraction efficiency.
When sugar cane is conveyed from one mill to the next mill, the cane blanket tends to expand as it is discharged from one mill and before it gets compressed and crushed by the next mill.
In order to improve the efficiency of the sugar juice extraction in the next mill, it is known to add water, or dilute juice to the sugar cane blanket as it expands, this process being known as "inhibition".
A disadvantage with adding water, or dilute juice at this stage in a two roll mill is that the sugar cane blanket is also required to be in a semi-compressed form. Pressure chutes are therefore positioned between each crushing stage. The pressure chutes maintain the sugar cane blanket or bagasse in a semi-compressed form as a consequence of the driving force necessary to transfer the bagasse to the next stage where the bagasse is transferred horizontally.
However, by maintaining the bagasse in a semi-compressed form, it does not fully expand and therefore inhibition is not totally successful.
When inhibition liquid is added to the semi-compressed bagasse, not only does the bagasse not absorb as much liquid as is possible if the bagasse was not maintained in a semi-compressed system, but because the bagasse is conveyed along a horizontal or slightly inclined angle, the inhibition liquid is sprayed or added to the top of the bagasse blanket and does not easily permeate through the blanket. It is generally not possible to spray or apply liquid to the bottom of the bagasse blanket as gravity effects will cause the spray or liquid to simply fall away and not be absorbed.
Another disadvantage with existing mills is the deleterious effect of upflow of sugar juice against the downflow movement of the sugar cane. For example, in the conventional three roll mills or the two roll mills where one roll is above the other roll, the sugar cane or bagasse moves along a generally horizontal pathway (although the pathway may be slightly inclined in particular areas). With the large pressures being exerted on the sugar cane between the crushing rolls, it is found the sugar juice has a tendency to move upward on to the face of the rolls and is therefore not efficiently separated from the cane. The upflow of juice also reduces the grip of the rolls on the bagasse by making the rolls slippery. The effect is a result of the substantially horizontal movement of the sugar cane or bagasse through the mill. Another disadvantage with the current mill arrangement is that if any pair of rolls has a mechanical fault, the entire tandem must stop operating as the defective stage cannot be bypassed.
Another disadvantage with existing mill arrangements is that the supply end of a pair of rolls has a fairly small feed zone which is the cross-section area where the bagasse begins to be pulled in by the rolls. It is desirable to have the feed zone as large as possible over which sugar cane or bagasse will self-feed into the rolls. Feed rolls are known to increase the cross-sectional area of the feed zone but add significantly to the cost of the equipment.
Another disadvantage with conventional mills is their high power consumption due to the large number of rolls required to provide an acceptable level of juice extraction.
Much attention has also been given to the design and manufacture of the crusher roll used in crushing sugar cane.
Crusher rolls are extremely well-known in the sugar cane industry and are widely used in sugar cane crushing mills to extract sugar cane juice from sugar cane, prior to clarification, evaporation and crystallisation of the sugar from the juice.
Much research and development has been undertaken to improve the efficiency of sugar juice extraction from the cane. The efficiency is measured in the power consumption required to drive the crusher rolls, the throughput of the sugar cane, and the extraction efficiency of the sugar juice from the cane.
It is known that the extraction efficiency can be improved by grooving the periphery of the crusher rolls, and by providing juice discharge channels behind the periphery. Examples of such rolls are found in the patent literature and the following patent documents exemplify the current rollers--Australian Patent Applications 74784/81, 84046/82, 34686/84, 10914/88; U.S. Pat. Nos. 3,536,002, 4,077,316, 4,220,288, 4,378,253, 4,168,660, 4,420,863, 4,804,418; United Kingdom Patent Application 2,025,260; French Patents 2,251,622, 2,569,608; German Patents 2,716,666, 2,657,232, 3,427,418.
The juice channels extend longitudinally inside the roll and just behind the peripheral or crushing surface of the roll. The juice channels are normally relatively small diameter tubes formed in the roll. These tubes are in fluid communication with the surface of the roll as is known in the art, and the function of the juice channels are to improve the separation of the liquid from the material to be crushed. The juice channels do not join each other and a fairly large number of parallel extending separating juice channels extend entirely around the roll.
The juice channels in turn communicate with an outlet such that juice can flow through the channel and into the outlet where in turn it is carried away for further processing. The outlet can be associated with a valve if desired. Outlets can be provided on both side faces of the roller, or on only one side face. The outlets are in sliding but sealing engagement with the side face of the roller such that as the roller rotates, the separate juice channels pass along the outlet to drain the juice. It is possible for the outlet to extend entirely around the roller, but it is more common for the outlet to extend only along a portion of the side face of the roller, this portion of course being where the juice passes into the juice channels.
It is known that crushing efficiency is improved by increasing the diameter of the rolls. However, the design and geometry of conventional 3, 4, 5 and 6 roll mills constrains the diameter of the rolls to about one half the length of the roll. This diameter to length ratio has been found to offer a reasonable compromise between capacity, extraction efficiency and roll weight.
The conventional thinking and teaching to improve the efficiency of the roller is by changing the peripheral groove shape, providing drainage channels in the roller, providing back pressure to the roller, and reducing the gap between the rollers.